It is already known in the prior art, for example, from DE-PS 934,407 to throw on to the impression cylinder the eccentric bushing mounted blanket cylinder of the printing unit of an offset press in two separate phases with a freely determinable interval between them. The blanket cylinder is first thrown on to the plate cylinder for pre-inking and only afterwards is thrown on to the impression cylinder. This is achieved by a cam-operated cam follower lever having a two-state pawl coupling. Throw-on and throw-off are effected when the grip edges of the cylinders are opposite one another. A disadvantage of arrangements such as these is the unfavorable mechanical dynamics, due to the harsh engagement of the pawl and the corresponding high driving torque which must be additionally provided by the main drive, particularly when the presses are running fast.
For two stage throw-on and throw-off of the blanket cylinder without the need for an abrupt input of torque from the main drive, it is known from DE 3,232,171 A1 and from DD 86,631 to produce the movement of the eccentric bushings by means of double-acting, fluid pressure working cylinders adapted to operate consecutively. Disadvantages here, however, arise from the very high constructional complexity and the accompanying mass of the components which are associated with the blanket cylinder bearing levers, and the fact that the operation of the working cylinders corresponds to a pure series or consecutive arrangement so that the force evolved by one cylinder always reacts on the other cylinder cooperating with it.
In U.S. Pat. No. 3,067,674 a throw-on/throw-off device is disclosed having a fluid pressure actuating cylinder which acts on a toggle transmission linkage, the latter in turn pivoting the eccentric bushings of the blanket cylinder. A disadvantage in this case is that a three-point working cylinder is necessary for two-stage throw-on and throw-off. Such a cylinder corresponds essentially to a series arrangement of two double-acting working cylinders and needs to provide substantial forces for modern high-speed presses to ensure that the blanket cylinder separates fast enough from the impression cylinder at high printing speeds in the event a sheet to be printed is missing.